reserve x for set,
  p,q,r,s,t,u for ExtReal,
  g for Real,
  a for Element of ExtREAL;

theorem Th21:
  ].p,q.[ c= ].p,q.]
proof
  let s;
  assume
A1: s in ].p,q.[;
  then
A2: p < s by Th4;
  s < q by A1,Th4;
  hence thesis by A2,Th2;
end;
